A technique for utilizing a plurality of antennas is used as one method for performing high-speed communication. Examples of the technique include a multiple input, multiple output (MIMO) technique for performing communication by using a channel which spatially increases due to a plurality of antennas and a beam-forming (BF) technique for increasing a reception signal gain in a specific direction by applying phase and magnitude weights of the respective antennas.
In a communication system using a plurality of antennas, all of the antennas may transmit the same signals to obtain an antenna diversity effect. In this case, when a broadcasting message is transmitted for example, there is a high possibility that a shadow zone is generated as shown in FIG. 1. Referring to FIG. 1, a distance between a shadow zone 100 and a transmission (Tx) antenna 110 and a distance between the shadow zone 100 and a Tx antenna 120 is different by a half-wavelength (i.e., λ/2). When the antenna-A 100 and the antenna-B 120 transmit the same signals to a receiving end 130, the receiving end 130 located in the shadow zone 100 receives two signals having a phase difference of 180 degrees. The two signals are received in a superimposed manner rather than being separately received. As a result, the receiving end 130 receives a null signal in which the two signals are offset from each other.
In order for the receiving end 130 located in the shadow zone 100 to receive a signal, a cyclic delay diversity (CDD) is used. A structure of a transmitting end using the CDD is shown in FIG. 2.
FIG. 2 is a block diagram illustrating a structure of a transmitting end in a conventional multiple-antenna wireless communication system. Although a transmission path of only one antenna is shown in FIG. 2, transmission paths of the remaining antennas are the same as that shown in FIG. 2.
Referring to FIG. 2, the transmitting end includes an inverse fast Fourier transform (IFFT) operator 201, a symbol rotator 203, a cyclic prefix (CP) adder 205, a digital-to-analog converter (DAC) 207, and a radio frequency (RF) transmitter 209.
The IFFT operator 201 transforms signals, which are input in parallel as many as the number of sub-carriers, into time-domain Tx symbols by performing an IFFT operation.
The symbol rotator 203 cyclically shifts the Tx symbols, which are received from the IFFT operator 201, by as many as the number of corresponding samples in a time domain. The cyclic shift in the time domain has the same meaning as a phase shift in a frequency domain as expressed by Equation 1 below.x(n) X(k)x(n−m) X(k)·e−j(2πkm/N)  [Eqn. 1]
In Equation 1, the left term denotes a time-domain signal and the right term denotes a frequency-domain signal. In addition, x(n) denotes an nth sample value in the time domain, X(k) denotes a kth sub-carrier value in the frequency domain, m denotes the number of samples shifted in the time domain, and N denotes the number of times of performing the fast Fourier transform (FFT).
That is, the symbol rotator 203 shifts a phase of the Tx symbol so as to compensate for a relative phase variation in a receiving end.
The CP adder 205 receives the shifted Tx symbols from the symbol rotator 203 and then adds a cyclic prefix so as to prevent a multiple-path propagation delay. In other words, the CP adder 205 adds a copy of last parts of the Tx symbols to front parts of the Tx symbols. The DAC 207 receives a digital signal from the CP adder 205 and converts the digital signal into an analog signal. The RF transmitter 209 receives a baseband signal from the DAC 207, converts and amplifies the baseband signal into an RF signal, and transmits the RF signal through an antenna.
According to the aforementioned structure, the transmitting end of the multiple-antenna communication system can prevent the generation of a shadow zone. In this case, if the cyclic shift in the time domain is performed as described above, phases of signals for all sub-carriers are shifted. If the communication system is a system capable of supporting multiple access, such as, an Orthogonal Frequency Division Multiple Access (OFDMA) system, one symbol is divided by the unit of sub-carrier and then is used by a plurality of receiving ends. That is, a receiving end that requires the cyclic delay diversity and a receiving end that does not require the cyclic delay diversity can simultaneously perform communication by using one symbol. For example, if there is a receiving end that uses a multiple-antenna scheme (e.g., space time coding, spatial multiplexing, etc.), a transmitting end that uses the cyclic delay diversity shifts a phase of a signal for a sub-carrier to which the cyclic delay diversity is not applied. As a result, throughput deterioration may occur in the receiving end when receiving a signal. Accordingly, there is a need for a cyclic delay diversity technique suitable for the multiple access communication system.